1. Technical Field
This invention relates to methods for processing video data, and in particular, to methods for encrypting video data for display on processor-based video systems.
2. Background Art
There is substantial interest in the computer and entertainment industries in incorporating video data into multi-media and related applications for use on processor-based video systems. Potential growth in this area has been enabled by the development of video compression schemes that reduce the bit rates required to display high quality video images and by the development of storage media, such as digital video discs (DVDs), which can accommodate data for an entire movie on a single disc.
Despite these advances in video compression and storage technologies, content providers have been reluctant to make their video works widely available for computer-based applications. This reluctance is attributable to the scant protection current processor-based video systems provide against the unauthorized acquisition and use of video data. For reasons discussed below, available encoding systems first compress and then encrypt video data prior to writing it to storage media. When the video data is subsequently decoded by a processor-based video system, the system must first decrypt and then decompress the video. This ordering of the decryption and decompression steps places video data on the processor system bus in compressed, unencrypted formats. In this state, it is a relatively simple matter to co-opt the video data for unauthorized uses.
The relative ordering of compression and encryption in the encoding process follows from the properties of the compression and encryption transforms conventionally employed. Compression is typically implemented on YUV-formatted video data in accordance with standards promulgated by the Moving Pictures Expert Group (MPEG, group ISO-IEC/JTC1 SC29/WG11) and the Joint Photographic Experts Group (JPEG, ISO/IEC International Standard 10918-1). Hereafter, MPEG is used to refer to MPEG-1 (ISO standard 11172), MPEG-2 (ISO standard 13818ISO), and JPEG compliant processes.
MPEG-compliant compression schemes reduce spatial and temporal redundancy in the video data by application of various lossy data transformations. Because lossy compression eliminates data from the video stream, it is commonly accepted that compressing encrypted data increases the likelihood that key data will be lost, preventing accurate decryption of the video data. It is also commonly accepted that encrypted data cannot be effectively compressed because encryption reduces the data redundancy on which compression methods operate. For these reasons, currently available encoding systems compress video data prior to encrypting it.
With the order of compression and encryption in the encoding process established by the above-considerations, the order of decompression and decryption in the decoding process is established by the non-commutation of the corresponding transformations. In particular, video data that is first compressed and then encrypted during encoding must be first decrypted and then decompressed during decoding. It is this ordering of the decryption and decompression processes that makes unencrypted video data available in both compressed and decompressed formats as it is transferred among the memory, video processor, and video rendering unit of conventional processor-based video systems. The sheer volume of uncompressed video data relative to compressed video data (100 G bytes versus 1.7 G bytes for a typical feature film) makes uncompressed data less useful to unauthorized users, despite its unencrypted state. However, the availability of unencrypted, compressed video data on the system bus severely compromises the ability of content providers to protect the integrity of their video products and control the uses to which it is put.
There is thus a need for video encoding and decoding systems that maintain data in a secure format at all points in the video decoding/display system where it is vulnerable to unauthorized acquisition.